In the manufacture of paper products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives applied in the wet end of the tissue making process. Two of the most important attributes imparted to tissue through the use of wet end chemical additives are strength and softness. Specifically for softness, a chemical debonding agent is normally used. Such debonding agents are typically quaternary ammonium compounds containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the material to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups, provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds in the sheet.
Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the tissue sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface.
Both debonding and creping increase levels of lint and slough in the product. Indeed, while softness increases, it is at the expense of an increase in lint and slough in the tissue sheet relative to an untreated control. It can also be shown that in a blended (non-layered) tissue sheet that the level of lint and slough is inversely proportional to the tensile strength of the tissue sheet. Lint and slough can generally be defined as the tendency of the fibers in the paper sheet to be rubbed from the sheet when handled.
A multi-layered tissue structure to enhance the softness of the tissue sheet. One such embodiment, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the product. The outer layers of such structures are composed of the shorter hardwood fibers, which may or may not contain a chemical debonder. A disadvantage to using layered structures is that while softness is increased the mechanism for such increase is believed due to an increase in the surface nap of the debonded, shorter fibers. As a consequence, such structures, while showing enhanced softness, do so with a trade-off of an increase in the level of lint and slough.
A chemical strength agent may be added in the wet-end to counteract the negative effects of the debonding agents. In a blended tissue sheet, the addition of such chemical strength agents reduces lint and slough levels. However, such reduction is done at the expense of surface feel and overall softness of the tissue sheet and becomes primarily a function of tissue sheet tensile strength. In a layered tissue sheet, strength chemicals are added preferentially to the center layer. While this perhaps helps to give a tissue sheet with an improved surface feel at a given tensile strength, such structures actually exhibit higher slough and lint at a given tensile strength, with the level of debonder in the outer layer being directly proportional to the increase in lint and slough. Co-pending U.S. patent application Ser. No. 09/736,924 (Shannon et al.) published on Aug. 22, 2002 discloses low slough tissue products made with acrylamides containing hydrophobic moieties. These synthetic polymers, while reducing the amount of slough compared to traditional debonders, still show an increase in slough with decreasing tensile strength.
Therefore there is a need for a means of reducing lint and slough in soft tissue sheets while maintaining the softness and strength of the tissue sheets. It is an objective of the present invention to design paper-making chemicals, more specifically tissue making chemicals, capable of reducing hydrogen bonding while also possessing ability to reduce lint and slough. It is a further objective to develop a process for making soft, low slough, low lint tissue products via wet end application of chemistry. It is a further objective of the present invention to make soft, absorbent, low lint and slough tissue products such as sanitary bath tissue, facial tissue, paper towels and the like via wet end application of such chemistry.